Faculty Profiles - OZAKI Nobuhiko

Published Papers - OZAKI Nobuhiko

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Growth of InAs/GaAs quantum dots with central emission wavelength of 1.05 μm using In-flush technique for broadband near-infrared light source

Yuji Hino, Nobuhiko Ozaki, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto （Part： Corresponding author )

Journal of Crystal Growth ( Elsevier B.V. ) 378 501 - 505 2013.09 [Refereed]

Self-assembled InAs quantum dots (QDs) emitting at around 1.05 μm in wavelength were grown on a GaAs substrate by using the In-flush technique. A rapid annealing (In-flush) process after the growth of the GaAs capping layer, which partially covers InAs-QDs, reduces the height of the InAs-QDs to the thickness of the capping layer. Using this approach, the emission wavelengths of the QDs were precisely controlled by varying the thickness of the capping GaAs layer. The central emission wavelength was suitably controlled in the range of approximately 1.22-0.95 μm. This method enables the realization of a broadband 1.05 μm light source with a bandwidth of beyond 200 nm via a combination of In-flushed QDs. Such a broadband light source of 1.05 μm in wavelength is applicable to optical coherence tomography (OCT), thereby enabling high-resolution and large penetration depth in the OCT images. In addition, we found that a higher emission intensity of the In-flushed QDs can be obtained by reducing the annealing temperature of the In-flush process. © 2013 Elsevier B.V. All rights reserved.

Monolithically grown multi-color InAs quantum dots as a spectral-shape-controllable near-infrared broadband light source

Nobuhiko Ozaki, Koichi Takeuchi, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Hisaya Oda, Kiyoshi Asakawa, Richard A. Hogg （Part： Lead author,　Corresponding author )

Applied Physics Letters ( AIP Publishing ) 103 ( 5 ) 051121 1-4 2013.07 [Refereed]

Enhancement upconversion luminescence in InAs-quantum dots embedded GaAs photonic-crystal slab line-defect waveguide

H. Oda, A. Yamanaka, N. Ozaki, N. Ikeda, Y. Sugimoto

2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013 ( IEEE Computer Society ) 2013 [Refereed]

Photonic crystals are very suitable for controlling radiation field and propagation characterization of light. As an important application, it is ultra compact and ultrafast optical integrated circuits (OIC) based on photonic crystals slab waveguide (PhC-WG) composing of the line-defects [1, 2]. The PhC-WG is also attractive for laser lasing, because very small group velocity of near the Brillouin zone (BZ) edge should enhance interactions between the radiation field and matter. Indeed, the lasing spectrum has been observed from optically pumped InAs-quantum dots (InAs-QDs) embedded PhC-WG [3]. And a small group velocity (Vg) in the PhC-WG gives rise to enhancement intensity and, therefore, is advantageous to nonlinear optical effect. In this work, we present the 1.55 μm to 1.3 μm upconversion luminescence (UL) based on two-photon absorption in InAs-QDs GaAs PhC-WG. © 2013 IEEE.

Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Ping Gu

Chemical Physics Letters ( Elsevier BV ) 552 102 - 107 2012.11 [Refereed]

Nanophotonics Based on Semiconductor-Photonic Crystal/Quantum Dot and Metal-/Semiconductor-Plasmonics

Kiyoshi ASAKAWA, Yoshimasa SUGIMOTO, Naoki IKEDA, Daiju TSUYA, Yasuo KOIDE, Yoshinori WATANABE, Nobuhiko OZAKI, Shunsuke OHKOUCHI, Tsuyoshi NOMURA, Daisuke INOUE, Takayuki MATSUI, Atsushi MIURA, Hisayoshi FUJIKAWA, Kazuo SATO

IEICE Transactions on Electronics ( Institute of Electronics, Information and Communications Engineers (IEICE) ) E95-C ( 2 ) 178 - 187 2012.02 [Refereed] [Invited]

Broadband Light Source Based on Four-Color Self-Assembled InAs Quantum Dot Ensembles Monolithically Grown in Selective Areas

Nobuhiko OZAKI, Koichi TAKEUCHI, Shunsuke OHKOUCHI, Naoki IKEDA, Yoshimasa SUGIMOTO, Kiyoshi ASAKAWA, Richard A. HOGG （Part： Lead author,　Corresponding author )

IEICE Transactions on Electronics ( Institute of Electronics, Information and Communications Engineers (IEICE) ) E95-C ( 2 ) 247 - 250 2012.02 [Refereed]

Formation of nanoparticles of organic molecules by liquid laser ablation

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Gu Ping

Materials Research Society Symposium Proceedings 1455 55 - 60 2012 [Refereed]

We carried out laser ablation of three organic molecules, rubrene (Rb), Oralith Brilliant Pink R (BP) and quinacridonequinone (QQ) in a poor solvent, water. As a result, nanoparticles of BP and QQ were formed, but those of Rb were not formed because of photodissociation. For a rigid molecule, QQ, optical properties of colloidal solutions were investigated in relation to the size of the included nanoparticles. A linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles was found, indicating that the nanoparticle diameter can be easily estimated from the absorption spectrum of a colloidal solution. From the solution, a nanoparticle film was fabricated on an electrode by the electrophoretic deposition method. © 2012 Materials Research Society.

Modification of optical response in quantum dots embedded in a photonic crystal waveguide via photonic band engineering

Nobuhiko Ozaki, Hayato Yoneda, Koichi Takeuchi, Hisaya Oda, Naoki Ikeda, Yoshimasa Sugimoto, Yoshinori Watanabe, Kiyoshi Asakawa （Part： Lead author,　Corresponding author )

Materials Research Society Symposium Proceedings 1438 13 - 18 2012 [Refereed]

We propose to use Purcell effect emerged at slow light regions in photonic crystal waveguide (PC-WG) modes for controlling the relaxation time of excited carriers in QDs. Straight GaAs PC-WGs including InAs-QDs with various lattice constants of PC were prepared in order to control the wavelength of the slow light in the PC-WG modes. PL measurements of the PC-WGs indicated enhancements of emission from QDs at the localized wavelength of slow light regions due to the Purcell effect. The enhanced emission peak wavelength was continuously shifted with the PC lattice constant. These results suggest that the PC-WG can be utilized to modify the spontaneous emission rate and carrier relaxation time of the embedded QD. This modification can be applied and useful for various QD-based optical devices as well as our proposed all-optical switching device based on PC-WG/QD. © 2012 Materials Research Society.

Multi-color quantum dot ensembles grown in selective-areas for shape-controlled broadband light source

N. Ozaki, K. Takeuchi, S. Ohkouchi, N. Ikeda, Y. Sugimoto, K. Asakawa, R. A. Hogg （Part： Lead author,　Corresponding author )

Journal of Crystal Growth ( ELSEVIER SCIENCE BV ) 323 ( 1 ) 191 - 193 2011.05 [Refereed]

Multi-color quantum dot (QD) ensembles were grown by selective-area growth method to realize a shape-controlled broadband light source. By using a metal-mask, QD ensembles and strain reducing layer (SRL) were formed in selective areas on a wafer. The SRL thickness was varied to achieve appropriate shifts in the peak wavelength of the QD emission spectrum up to 90 nm. A summation of PL spectra obtained from the multi-color QD ensembles shows a broadband emission spectrum with a width of approximately 120 nm, even though this spectrum is attributed to the ground state emissions of these QD ensembles. A current-induced broadband light source such as a superluminescent diode (SLD) based on the multi-color QD ensembles is expected to have an emission spectrum with a width of more than 120 nm owing to the combination of excited state emissions. Furthermore, a desired shape of the SLD spectrum can be obtained by controlling the injection current applied to each QD ensemble. This approach is promising for a shape-controlled broadband SLD, and it is particularly applicable to optical coherence tomography (OCT). (C) 2010 Elsevier B.V. All rights reserved.

InAs quantum-dots laser utilizing GaAs W1 type photonic-crystal slab line-defect waveguide

H. Oda, N. Hamada, A. Yamanaka, N. Ozaki, N. Ikeda, Y. Sugimoto

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 CB. P.4 THU 2011

Photonic crystals are very suitable for controlling radiation field and propagation characterization of light. As an important application, it is ultra compact and ultrafast optical integrated circuits (OIC) based on photonic crystals slab waveguides (PC-WGs) composing of the line-defects [1, 2]. The PC-WG is also attractive for laser lasing, because very small group velocity of near the Brillouin zone (BZ) edge should enhance interactions between the radiation field and matter. Indeed, the lasing spectrum has been observed from optically pumped InAs-quantum dots (InAs-QDs) embedded in PC-WG of the multimode W3 type (three rows missing line-defect) [3]. In the present study, we observe laser action in InAs-QDs PC-WG of the single-mode W1 type (single row missing line-defect). © 2011 IEEE.

WIDEBAND OPERATION OF 2D PHOTONIC CRYSTAL DIRECTIONAL COUPLER WITH TOPOLOGY OPTIMIZED WAVEGUIDE BENDS

YOSHINORI WATANABE, NAOKI IKEDA, YOSHIAKI TAKATA, YOSHINORI KITAGAWA, NOBUHIKO OZAKI, YOSHIMASA SUGIMOTO, KIYOSHI ASAKAWA

Journal of Nonlinear Optical Physics & Materials ( World Scientific Pub Co Pte Lt ) 19 ( 04 ) 543 - 550 2010.12 [Refereed]

Nanophotonics technology for advanced quantum dot / photonic crystal device and metal / semiconductor plasmonic device

Y. Sugimoto, N. Ikeda, D. Tsuya, N. Ozaki, H. Oda, A. Yamanaka, A. Miura, T. Nomura, D. Inoue, H. Fujikawa, S. Ohkouchi, Y. Watanabe, Y. Koide, K. Satoh, K. Asakawa

2010 Photonics Global Conference, PGC 2010 1月5日 2010

Nano-photonic structures/materials such as surface plasmon and negative-index material as well as quantum dots (QD) and photonic crystals (PCs) are discussed from potentials for new photonic devices and technologies. PC slab waveguides and QDs were developed for key photonic devices such as ultra-small and ultrafast symmetrical Mach-Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF). We have studied energy-saving light/matter interaction in a PC waveguide with emphasis on virtual-excitation-based nonlinear effects such as optical Kerr effect. We have also demonstrated a new RGB color filter on Al film with sub-wavelength holes in a large area with a high precision and optical beam-steering device with a wide steering angle.

Evolution of nanophotonics from semiconductor photonic crystal device to metal/semiconductor plasmonic device

K. Asakawa, Y. Sugimoto, N. Ikeda, D. Tsuya, Y. Koide, Y. Watanabe, N. Ozaki, D. V. Kumar, T. Nomura, D. Inoue, A. Miura, H. Fujikawa, K. Sato

Conference Proceedings - 5th International Conference on Advanced Optoelectronics and Lasers, CAOL' 2010 18 - 24 2010 [Refereed]

This paper reviews our recent activities on nanophotonics based on a photonic crystal (PC)/quantum dot (QD)-combined structure for an all-optical device and a metal/semiconductor composite structure using surface plasmon (SP) and negative refractive index material (NIM). The former structure contributes to an ultrafast signal processing component by virtue of new PC design and QD selective-area-growth technologies, while the latter provides a new RGB color filter with a high precision and optical beam-steering device with a wide steering angle. © 2010 IEEE.

Monolithic Fabrication of Two-Color InAs Quantum Dots for Integrated Optical Devices by Using a Rotational Metal Mask

Nobuhiko Ozaki, Shunsuke Ohkouchi, Yoshiaki Takata, Naoki Ikeda, Yoshinori Watanabe, Yoshimasa Sugimoto, Kiyoshi Asakawa （Part： Lead author,　Corresponding author )

Japanese Journal of Applied Physics ( IOP Publishing ) 48 ( 6 ) 065502 1-4 2009.06 [Refereed]

Sequential Operations of Quantum Dot/Photonic Crystal All-Optical Switch With High Repetitive Frequency Pumping

Y. Kitagawa, N. Ozaki, Y. Takata, N. Ikeda, Y. Watanabe, Y. Sugimoto, K. Asakawa

Journal of Lightwave Technology ( Institute of Electrical and Electronics Engineers (IEEE) ) 27 ( 10 ) 1241 - 1247 2009.05 [Refereed]

Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Y. Sugimoto, N. Ikeda, N. Ozaki, Y. Watanabe, S. Ohkouchi, T. Kuroda, T. Mano, T. Ochiai, K. Kuroda, N. Koguchi, K. Sakoda, K. Asakawa

MICROELECTRONICS JOURNAL ( ELSEVIER SCI LTD ) 40 ( 4-5 ) 736 - 740 2009.04 [Refereed]

Two types of nanophotonic technologies-two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)-were developed for key photonic device Structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach-Zehnder (SMZ)type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements. (C) 2008 Elsevier Ltd. All rights reserved.

Site-controlled InAs quantum dot formation grown on the templates fabricated by the Nano-Jet Probe method

S. Ohkouchi, N. Ozaki, Y. Sugimoto, H. Ishikawa, K. Asakawa

JOURNAL OF CRYSTAL GROWTH ( ELSEVIER SCIENCE BV ) 311 ( 7 ) 1819 - 1821 2009.03 [Refereed]

We have developed a nanoprobe-assisted bottom-up technique that allows to fabricate site-controlled quantum dots (SC-QDs), using a specially designed atomic force microscope probe, referred to as the Nano-jet Probe (NJP). Beginning with the fabrication of regular QD arrays by using the NJP, we vertically aligned self-assembled InAs QDs using the strain-induced stacking method and produced assembled QD stacks. In order to improve the optical property of the SC-QDs, an AlGaAs barrier layer was introduced in the spacer layer of the stacked structures. The photoluminescence measurements of the fabricated structures revealed that they had good crystallographic qualities. (C) 2008 Elsevier B.V. All rights reserved.

Nanophotonic technologies for innovative all-optical signal processor using photonic crystals and quantum dots

Y. Sugimoto, N. Ikeda, N. Ozaki, Y. Watanabe, S. Ohkouchi, S. Nakamura, K. Asakawa

TRANSPORT AND OPTICAL PROPERTIES OF NANOMATERIALS ( AMER INST PHYSICS ) 1147 179 - + 2009 [Refereed]

GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs quantum dots (QDs) were developed for key photonic device structures in the future. An ultrasmall and ultrafast symmetrical Mach-Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip-flop device (PC-FF) have been developed based on these nanophotonic structures for an ultrafast digital photonic network. To realize these devices, two important techniques were developed. One is a new simulation method, i.e., topology optimization method of 2DPC WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another is a new selective-area-growth method, i.e., metal-mask molecular beam epitaxy method of InAs QDs. This technique contributes to achieving high-density and highly uniform InAs QDs in a desired area such as an optical nonlinearity-induced phase shift arm in the PC-FF. Furthermore, as a unique site-controlled QD technique, a nano-jet probe method is also developed for positioning QDs at the centre of the optical nonlinearity-induced phase shift arm.

Photonic crystal and quantum dot technologies for ultra-small and ultra-fast all-optical flip-flop

N. Ozaki, N. Ikeda, Y. Watanabe, Y. Takata, Y. Kitagawa, S. Ohkouchi, S. Nakamura, A. Watanabe, A. Mizutani, Y. Sugimoto, K. Asakawa （Part： Lead author )

ECS Transactions 16 ( 41 ) 31 - 37 2009 [Refereed]

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides and InAs quantum dots (QDs) are reviewed for symmetric Mach-Zehnder type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and optical flip-flop device (PC-FF). In the 1st phase of this work, ultra-fast (∼ps) and ultra-low energy (∼100 fJ) switching has been demonstrated using the PC-SMZ chip with 600 μm x 300 μ in size. In the 2nd phase, the concept of the PC-FF based on the dual PC-SMZs for providing a latch function has been proposed for a future ultra-fast optical digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization method of the 2DPC waveguide with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop a selective-area-MBE growth technique with a metal-mask method for high-density and highly uniform InAs QDs with different absorption wavelengths in different areas. ©The Electrochemical Society.

Topology optimization of a wavelength-selective Y-junction for 2D photonic crystal waveguides

Yoshinori Watanabe, Naoki Ikeda, Yoshiaki Takata, Yoshinori Kitagawa, Nobuhiko Ozaki, Yoshimasa Sugimoto, Kiyoshi Asakawa

JOURNAL OF PHYSICS D-APPLIED PHYSICS ( IOP PUBLISHING LTD ) 41 ( 17 ) 2008.09 [Refereed]

The topology optimization ( TO) technique has been employed to design a wavelength-selective Y-junction for two-dimensional photonic crystal slab waveguides. The TO design exhibits high-pass and low-pass filtering spectra for two different paths in the Y-junction. Large cross-talk suppression of -20 dB at wavelength separation of 30 nm has also been obtained. These calculated characteristics have been experimentally verified by using a GaAs-based air-bridge-type photonic crystal waveguide. The TO-designed Y-junction contributes to an essential component for a new photonic crystal-based all-optical flip-flop, PC-FF.

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